# P0440: Evaporative Emission System **Verdict:** Safe to drive Emissions - Generic OBD-II code, applies to all gasoline cars and light trucks sold in the US since 1996. - **Typical cost:** $20-$600 - **Urgency:** Keep driving - **Most likely:** Gas cap, EVAP hose, or valve problem - **DIY?:** Steps 1-2 ## Quick answer P0440 means the engine computer has found a general fault in the evaporative emission (EVAP) system, which traps gasoline vapor from the fuel tank and burns it in the engine instead of letting it escape. It is a broad code that can mean a leak, a stuck valve, or an electrical problem in the system. A loose gas cap, a cracked hose, or a failed purge or vent valve are common causes. ## What does P0440 mean? The evaporative emission (EVAP) system keeps gasoline vapor from escaping into the air. Vapor from the fuel tank flows into a charcoal canister, which soaks it up. When the engine is warm and running, the computer opens a purge valve so intake vacuum pulls that stored vapor into the engine to be burned. A vent valve lets fresh air into the canister and can close to seal the whole system. To check for leaks and stuck valves, the computer seals the system and watches a fuel tank pressure sensor, or on some vehicles uses a small pump or the natural pressure change as the tank cools after shutdown. P0440 is the broad, general EVAP malfunction code. Many cars set a more specific code instead, but some use P0440 for faults that do not fit another one. The test usually needs a fuel level between roughly a quarter and three-quarters full and suitable temperatures, and most cars light the check engine lamp after two failed tests. ## What are the symptoms of P0440? - Check engine light on, car drives normally - Faint fuel smell near the car, especially after parking - Failed emissions test - Sometimes hard to fill the fuel tank ## What causes P0440? Ranked from most to least likely. Check them in this order: the cheap, common causes first. 1. **Loose, damaged, or wrong gas cap** (most common). A cap that is not tightened, has a cracked or hardened seal, or is a poor-fitting aftermarket part lets vapor escape, and the system fails its seal test. It is the cheapest and quickest check, so always start here. Capless filler systems can leak too if the flap seal is dirty or damaged. 2. **Cracked or disconnected EVAP hose** (common). The rubber and plastic hoses that run between the tank, charcoal canister, purge valve, and engine harden and crack with heat and age. Connectors can also break or pop off after other work nearby. A smoke test usually finds these quickly, though some leaks hide under the car near the tank, where they are hard to see. 3. **Faulty purge or vent valve** (common). The purge valve near the engine and the vent valve near the tank or canister must both open and close on command. Either one can stick open, stick closed, or leak, often because of dirt, moisture, or charcoal dust. A stuck valve keeps the system from sealing for its test or from purging vapor properly. 4. **Damaged charcoal canister or filler neck** (less common). The charcoal canister can crack from impact or get saturated with liquid fuel from overfilling. A steel filler neck can rust through where it is exposed to road spray, which is more common in states that salt their roads. Both usually sit near the tank, out of sight, and tend to show up on a smoke test. 5. **Faulty fuel tank pressure sensor** (rare). The fuel tank pressure sensor reports how well the system holds pressure or vacuum during the leak test. If it drifts or fails, the system can appear to leak or fail to purge when it is actually fine. This is rare, and many of these sensors sit on top of the tank, which can make replacement labor-intensive. ## How do you diagnose P0440? 1. Check the gas cap, tighten it until it clicks, and look at the seal for cracks. 2. Look for more specific EVAP codes like P0441, P0442, P0446, or P0455 that narrow the search. 3. Have a shop run a smoke test, which pushes harmless smoke into the system so leaks become visible. 4. Command the purge and vent valves with a scan tool to see if they open and close. ## How do you fix P0440, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Replace gas cap | $20-$50 | Use an original-equipment or good-quality cap made for your vehicle. | | Replace EVAP hose or line | $100-$350 | Depends on where the hose is and how much has to come off to reach it. | | Replace purge or vent valve | $120-$400 | Fixes a valve that is stuck or not sealing. | Full price breakdowns: [evap purge valve replacement cost](/cost/evap-purge-valve-replacement). ## Is it safe to drive with P0440? > **Safe to drive.** It is safe to drive. EVAP problems mostly affect emissions, but if you smell strong fuel odor, get it checked soon, since a fuel leak is a fire risk. ## Will P0440 fail an emissions test? With P0440 stored and the check engine light on, a car fails an OBD-based emissions test in most states. The EVAP monitor is often the slowest to complete after a code is cleared. Many states allow one incomplete monitor on 2001 and newer cars, and EVAP is commonly the one left, but rules vary by state. ## After the repair: clearing P0440 After the repair, clear the code and drive normally for several days. The EVAP test usually needs a cold start after the car sits for hours, a fuel level between about a quarter and three-quarters full, and mild outdoor temperatures. The repair worked when the EVAP monitor shows ready on a scan tool and no pending code returns. ## How to prevent P0440 - Tighten the gas cap until it clicks every time you refuel, and replace it if the seal cracks. - Stop filling at the first pump click, since topping off can flood the charcoal canister with fuel. - Use an original-equipment or quality replacement gas cap made for your exact vehicle. - Rinse road salt and mud from under the car in winter to slow rust on the filler neck. - Have a faint fuel smell checked early, before a small leak grows or a part fails completely. ## Codes often seen with P0440 - [P0441](/codes/p0441): Evaporative Emission System Incorrect Purge Flow - [P0442](/codes/p0442): Evaporative Emission System Leak Detected (small leak) - [P0446](/codes/p0446): Evaporative Emission System Vent Control Circuit - [P0455](/codes/p0455): Evaporative Emission System Leak Detected (large leak) - [P0456](/codes/p0456): Evaporative Emission System Leak Detected (very small leak) ## Frequently asked questions ### Can I drive with a P0440 code? Yes, P0440 does not affect how the car drives or harm the engine. Get it fixed before your emissions test, and have it checked sooner if you smell raw fuel. A strong smell means fuel or vapor is escaping, which is a fire risk and not just an emissions problem. ### Can a loose gas cap cause P0440? Yes, a loose or worn gas cap is one of the most common causes of P0440. Tighten it until it clicks, or replace it if the seal is cracked. Then give the car several days of normal driving, because the computer must rerun its EVAP test before the light goes out. ### What is a smoke test? A smoke test is how shops find EVAP leaks, by filling the system with harmless smoke at very low pressure and watching where it comes out. It usually costs about $100 to $180 and saves guessing on parts. It can reveal leaks too small to see or smell during a normal inspection. ### How much does it cost to fix P0440? Most P0440 repairs cost $20 to $600 at an independent shop. A new gas cap is $20 to $50, an EVAP hose $100 to $350, and a purge or vent valve $120 to $400. A smoke test to find the leak usually adds about $100 to $180 if the cause is not obvious. ### Will P0440 make me fail an emissions test? Yes, a car with P0440 and the check engine light on fails an OBD-based emissions test in most states. Clearing the code resets the EVAP monitor, which can take days to complete. Many states accept one incomplete monitor on 2001 and newer cars, but rules vary, so check yours. ### Can I fix P0440 myself? You can often fix P0440 yourself if the cause is the gas cap or an obvious cracked hose near the engine. Purge valves are also easy to replace on many cars. Finding hidden leaks under the car usually needs a shop smoke test, and parts mounted on the tank may require lowering it. ### What is the EVAP system on a car? The EVAP system traps gasoline vapor from the fuel tank so it does not escape into the air. A charcoal canister stores the vapor, and the engine burns it later through a purge valve. The system includes the gas cap, filler neck, hoses, canister, purge and vent valves, and a pressure sensor. ### What is the difference between P0440 and P0455? P0440 is a general EVAP system fault that can be a leak, a stuck valve, or an electrical problem. P0455 specifically means a large leak was found. Many cars set the more specific code when they can, so P0440 on its own sometimes needs more testing to pinpoint the cause. ### Can P0440 cause a fuel smell? Yes, P0440 can come with a faint fuel smell, especially near the rear of the car after parking or around the gas cap. A small leak often has no smell at all. A strong or growing smell, or any liquid fuel under the car, needs checking right away because of the fire risk. ### Why does P0440 come back after replacing the gas cap? It returns because the cap was not the leak, or not the only one. Cracked hoses, a leaking purge or vent valve, a damaged canister, or a rusty filler neck can all set it. A smoke test is the fastest way to find the real cause instead of replacing parts one by one. ## Sources - [SAE J2012: Diagnostic Trouble Code Definitions](https://www.sae.org/standards/content/j2012_201812/) - [EPA: On-Board Diagnostics (OBD)](https://www.epa.gov/state-and-local-transportation/vehicle-emissions-board-diagnostics-obd) ## Other emissions codes - [P0430 Catalyst System Efficiency Below Threshold (Bank 2)](https://diagxcar.com/codes/p0430) - [P0431 Warm Up Catalyst Efficiency Below Threshold (Bank 2)](https://diagxcar.com/codes/p0431) - [P0441 Evaporative Emission System Incorrect Purge Flow](https://diagxcar.com/codes/p0441) - [P0442 Evaporative Emission System Leak Detected (small leak)](https://diagxcar.com/codes/p0442) - [P0443 Evaporative Emission System Purge Control Valve "A" Circuit](https://diagxcar.com/codes/p0443) - [P0446 Evaporative Emission System Vent Control Circuit](https://diagxcar.com/codes/p0446) - [P0449 Evaporative Emission System Vent Valve/Solenoid Circuit](https://diagxcar.com/codes/p0449) - [P0451 Evaporative Emission System Pressure Sensor/Switch "A" Circuit Range/Performance](https://diagxcar.com/codes/p0451) - [P0452 Evaporative Emission System Pressure Sensor/Switch "A" Circuit Low](https://diagxcar.com/codes/p0452) - [P0455 Evaporative Emission System Leak Detected (large leak)](https://diagxcar.com/codes/p0455) - [P0456 Evaporative Emission System Leak Detected (very small leak)](https://diagxcar.com/codes/p0456) - [P0457 Evaporative Emission System Leak Detected (fuel cap loose/off)](https://diagxcar.com/codes/p0457) - [P0496 Evaporative Emission System High Purge Flow](https://diagxcar.com/codes/p0496) - [P0498 Evaporative Emission System Vent Valve Control Circuit Low](https://diagxcar.com/codes/p0498) ## Symptoms - [Why does my car smell like gas?](https://diagxcar.com/symptoms/car-smells-like-gas) ## Repair costs - [EVAP purge valve replacement cost](https://diagxcar.com/cost/evap-purge-valve-replacement): $120-$350 --- Source page: https://diagxcar.com/codes/p0440 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.